1. Field of the Invention
This invention relates generally to the technology of mooring vessels such as oil tankers, floating barges, FPSOs (floating, production, storage and offloading vessels), and the like, to a station where fluids may be transferred from a producing well or storage area to the vessel. In particular, the invention relates to a single point mooring system for vessels on a body of water.
2. Background Art
Single point mooring systems have frequently been used in offshore locations for the loading and unloading of hydrocarbons or other flowable cargos into or out of marine vessels such as tankers, FPSOs, barges and the like. Many such systems have been developed and are now in use for both loading and unloading hydrocarbons at offshore locations. Examples of such conventional systems include the catenary anchor leg mooring (CALM) apparatus and the single anchor leg mooring (SALM) apparatus.
The catenary anchor leg mooring system (CALM) holds a floating buoy by an array or pattern of anchors attached by mooring chains or lines. The vessel that is to be loaded is attached to the floating buoy by suitable mooring hawsers or a rigid mooring arm. In such a mooring system the position restoring mooring forces are provided by the horizontal force component in the mooring chains. However, in shallow water it has been difficult to arrange the mooring chains with sufficient slack to provide adequate resiliency or horizontal spring in the mooring system. In such instances, the horizontal movement of a moored vessel caused by external forces due to waves, winds and currents can cause sudden extremely large forces which may exceed the capacity of the mooring system to maintain the vessel in the hydrocarbon transferring mode.
Conventional single anchor leg mooring systems (SALM) have used one floating mooring buoy attached with a suitable chain or articulated arm to a lower base structure fixed to the sea bottom or floor. By adjusting buoy draft, a desired tension level is created in the anchor leg. The tension provides a constant restoring or horizontal urging force for resiliently mooring a floating vessel to the buoy. In shallow water the vessel position restoring capacity of the SALM mooring system is also reduced and not suitable for mooring large vessels. In general, the conventional SALM system has also not been particularly well suited for use in a location or region where large ice flows may contact or may be expected to impact the mooring buoy. Accordingly, a completely submerged mooring system that provides resiliency and restoring forces to a moored vessel is desirable.
U.S. Pat. No. 4,530,302 issued to Pedersen and U.S. Pat. No. 4,825,797 issued to Polfervaart et al., both incorporated herein by reference, disclose submerged single point mooring systems having a submerged mooring base on which a submerged yoke is pivotably mounted. The forked end of the yoke carries heavy weights and is connected to the vessel by way of vertical or near vertical suspension members connected to the bow of the vessel. Universal joints and/or triaxial articulation joints are included in the mooring system to allow the moored vessel to weathervane, roll, pitch and heave under the effects of current, wave, wind and tides. The heavy weights provide a self-actuating restoring force to the vessel.
However, the known submerged single point mooring systems of prior art do not readily lend themselves to adjustment for the widely varying depths of the shallow water mooring locations and are thus not particularly suitable for relocation from one mooring location to another.